The present invention relates to attaching a plate to a rotatable drum and, more particularly, to conveniently attaching plates of various sizes to the drum of a so-called external-drum imagesetter or platesetter, by means of movable dynamic clamps, such that enable it to operate even at very high rotation speeds.
Imagesetters, namely marking engines for plotting images on plates or sheets of film, of the so-called external-drum type, are known in the art. Typically, a sheet is attached to the outer surface of a rotating drum and held in contact with the surface by a vacuum system, which has orifices distributed over the surface. Often, such an imagesetter is required to handle sheets of various sizes. U.S. Pat. No. 5,383,001 to Bosy addresses such a need, by providing a suitable design of the vacuum system.
External-drum platesetters are a particular category of external-drum imagesetters, wherein the image is plotted on a printing plate. Printing plates are generally made of metal and therefore they are stiffer than films and their mass per unit area is much higher. Thus, when a plate is attached to the drum, it requires a greater force to keep it in contact with the drum""s surface at any given rotational speed, to counteract the centrifugal force, than is required by a film. Consequently, a vacuum system to hold plates on the rotating drum must be more efficient and more powerful than that used for holding films. However, even with a powerful vacuum system, there is a danger of failure, due to a bend or a bulge at the edge of the plate or due to a failure in the vacuum system itself. Moreover, above a certain rotational speed the centrifugal force per unit area may exceed the difference between atmospheric pressure and the actual vacuum pressure (which, in turn, has a practical lower limit), thus rendering such a plate attachment system, solely based on vacuum, ineffectual. In such a system failures may cause the entire plate to fly off the drum by centrifugal actionxe2x80x94which would be disastrous for the entire machine. Therefore platesetters require mechanical attachment meansxe2x80x94either to assist the vacuum system and guard against drastic failure, or as the main holding means, possibly still aided by vacuum. One common and useful such means is clamps mounted on the drum and operative to hold down the edges of the plate.
Clamps and grippers for holding a plate onto a rotatable drum have been known in the art. For example, clamps (also called grippers) are generally used in offset printing presses to hold the printing plate onto a cylinder. During operation, centrifugal forces act to cause a pulling force on each edge of the plate, such that tends to pull the edge from the clamp. Therefore in printing presses, clamps are designed to exert enough gripping force to exceed that required for countering the centrifugally induced pulling force, at maximum expected rotational speed, by a safety margin. Additionally, the gripping force of the clamps must exert a positive stretching force on the plate, in order to assure contact between the plate and the cylinder over its entire surface, as well as to counter the drag forces exerted on the plate by external rollers and cylinders that are in contact with it during the printing operation. Such clamps are disadvantageously cumbersome for mounting on platesetters and, moreover, difficult to activate (i.e. to open for accommodating the plate), because they must include either a heavy bolting or leveraging mechanism or a very strong spring-loading mechanism.
U.S. Pat. No. 3,203,074 describes an improved clamp, in which the gripping and stretching forces increase by centrifugal action in proportion to the rotational speed. A clamp with such a centrifugal action (also termed dynamic clamp) may overcome the disadvantages discussed in the previous paragraph. U.S. Pat. No. 4,250,810 discloses a further improved clamp with centrifugal action, whereby the gripping and stretching forces can be independently adjusted.
It is noted that in platesetters, there usually is also a vacuum system, which assures contact between the plate and the drum over its entire surface, and that therefore no stretching force, but only a gripping force, is required at the edges of an attached plate. U.S. Pat. No. 5,335,046 discloses a clamp for the drum of a platesetter with a mechanism that centrifugally induces a gripping force. There is a row of such clamps for the leading edge and another one for the trailing edge. The mechanism includes a weight, connected to the clamp by a rod, located inside the drum near the opposite surface. The clamp may be opened by means of a piston and cylinder assembly, activated by supplied pressurized air. The clamp mechanism of the ""046 patent has two serious drawbacks: (1) It is relatively complicated, with a relatively large number of parts, and thus is expensive to manufacture. (2) It does not conveniently accommodate plates of various sizes. The secondly listed drawback also applies to other types of clamps of prior art, such as those discussed hereabove. The ""046 patent does disclose the possibility of providing a row of trailing edge clamps for each plate size; however, such an arrangement is even more expensive and, furthermore, does not accommodate small variations in plate size; moreover, it does not provide for mounting a plurality of plates peripherally around the drum, in the case of small plates, thus detracting from productivity.
Associated with the manner of clamping plates to the drum is the manner of loading them onto the drum and of unloading them following the plotting operationxe2x80x94whether manually or by automatic mechanisms. Most systems of prior art are limited in the loading- and unloading configurations practically possible, generally enabling loading only upon, say, clockwise rotation of the drum and, correspondingly, unloadingxe2x80x94only upon counterclockwise rotation of the drum. This places constraints on the design of loading- and unloading mechanisms and on their placement within the machine. More freedom in such design and placement may, in many cases, be advantageous.
There is thus a widely recognized need for, and it would be highly advantageous to have, a clamping system, for attaching a plate to a rotatable drum, that is adaptable to any plate size, possibly allowing the attaching of multiple plates, of various sizes, and that enables loading and unloading plates upon rotation of the drum in either direction.
The present invention successfully addresses the shortcomings of the presently known configurations by providing a an inexpensive dynamic clamping system, for attaching plates of various sizes to a rotatable drum, such as that of a platesetter, so that they are firmly gripped at any rotational speed.
The present invention discloses a novel design of a clamp, which allows placing it at any position around the drum, to accommodate any length of plate, and to be anchored at that position with a force that increases by centrifugal action. The design of the clamp is, moreover, such that the gripping force exerted by it on the edge of the plate likewise increases with the rotation speed of the drum by centrifugal action. The present invention further discloses a novel way of using such clamps to load and unload plates, to and from the drum, in the same general direction, thus enabling more practical configurations for feeding and disposing plates.
More specifically and according to a preferred embodiment, the clamp of the present invention comprises a gripper that is attached to the drum by means of a hinge. The largest part of the gripper consists of a body that extends substantially to one side of the pivot axis of the hinge. The portion of the gripper that extends to the other side of the pivot axis is relatively short and at its end is a tip, operative to press an edge of the plate onto the underlying surface of the drum. A spring is operative to provide a static force for the tip""s pressure on the plate. During drum rotation, centrifugal force virtually acting on the large body of the gripper, in a radially outward direction, is translated by the pivoting effect into a radially inward force on the tip, thus increasing its pressure on the plate.
In a slidable configuration, the clamp also comprises a base, attached to the other member of the hinge (the first member being attached to, or part of, the gripper). The drum is provided with a plurality of undercut grooves, preferably in a circumferential orientation, each groove being wider at its bottom, inside the wall of the drum, than near the drum""s surface. The base of the clamp is shaped to fit the wide portion of the groove and to slide therealong. The gripping force of the clamp also acts to push the base against the upper surface of the groove""s wide portion, causing friction therebetween, thus keeping the clamp from sliding along the groove. The force of this friction also increases with the drum""s rotation speed, owing to the increased gripping force and owing to centrifugal action on the base itself.
Slidable dynamic clamps are preferably mounted in pairs around each groove, there being a plurality of grooves across the length of the drum; the clamps of each pair face each other so as to clamp opposite edges of a plate. Thus each pair of clamps along a groove can clamp down a plate, its length being variable over a wide range. Likewise, a plurality of plates, of various and variable widths, may be mounted across the length of the drum, each plate clamped down by two or more pairs of clamps on adjacent grooves.
To mount a plate on the drum, a first clamp of each relevant pair is first opened, by pushing on its body radially inward by means of a stationary activation mechanism, then engaging it to the leading edge of the plate, which is fed from a suitably positioned loading mechanism, and releasing the clamps to grip it. The drum is then rotated, say clockwise, to pull the plate and wrap it around the drum. Then the other clamp of each pair is opened and the drum is rotated backward (say counter-clockwise), while the clamp remains stationary, until the trailing edge of the plate is engaged by the clamp, whereupon it is released, thus gripping that edge by slidable clamps. After normal imaging operation, the plate is demounted, in the same general order, by first releasing the first clamp of each pair (which grips the leading edge of the plate) and moving it away from the plate (by say counter-Clockwise rotation of the drum), thus freeing that edge, then rotating the drum forward (Clockwise), thus pushing the plate onto a suitably position unloading bin, and finally releasing the second clamp of the pair, thus freeing the plate.
According to the present invention, there is provided apparatus for clamping flexible plates of various sizes onto the cylindrical surface of a rotatable drum, comprising:
a plurality of grooves, cut into the surface of the drum; and
at least one clamp, each attached to the drum at one or more corresponding grooves and being switchable between two statesxe2x80x94a clamping state and a released state;
the clamp being fixed at any of a plurality of positions along the respective grooves and operative to clamp an edge of the plate, which crosses the groove, to the surface of the drum, when in a clamping state, and being slidable along the respective groove, when in a released state.
According to further features of the preferred embodiments of the invention, each of the clamps is a dynamic clamp, whereby the force at which it clamps the plate, when in the clamping state, consists of a constant component and a dynamic component, which is derived from centrifugal action and is essentially proportional to the rotational speed of the drum. Furthermore, each of the clamps includes:
a base that is slidable within, and along, the respective groove and forming one member of a hinge, having a pivot axis, and
a gripper, formed to have a tip and operative to press the edge of the plate against the drum,
the gripper being attached to the base so as to form the other member of the hinge and so that the center of mass of the gripper is at a substantial distance from the pivot axis and in a first direction therefrom that is generally opposite the direction of the tip from the pivot axis.
Additional features of the present invention include an activation mechanism, not attached to the drum and including at least one member that is movable essentially radially to the cylindrical surface and operativexe2x80x94
to engage the gripper of any clamp, while in an appropriate position, at a point thereof that lies in the first direction from the pivot axis,
to switch the clamp to the release state, by moving radially inwards, and
to switch the clamp to the clamping state, by moving radially outwards.
In a preferred embodiment, the grooves lie along circumferential lines and the activation mechanism and any of the clamps, while in the release state and while being engaged by a respective one of the members, cooperate to cause the base of the clamp to slide along the respective groove in reaction to a rotation of the drum.
According to yet other features of the invention, the number of the grooves is at least four, the apparatus being operative to clamp at least two plates, side by side, onto the surface of the drum, the plates not necessarily being equal in length or in width, and the number of the clamps in any groove is even, the clamps forming circumferentially adjacent pairs, the clamps of each pair being disposed and operative to clamp respective opposite edges of a corresponding plate.
In an alternative configuration of the invention, some or all of the grooves lie along lines parallel to the axis of the drum.
In another aspect of the present invention, there is provided apparatus for loading a flexible plate onto the cylindrical surface of a rotatable drum, attaching the plate thereto and unloading the plate therefrom, the plate having a leading edge and a trailing edge, the apparatus comprising:
a plate loading assembly and a plate unloading assembly, both positioned in the vicinity of the drum; and
at least one pair of sets of clamps, attached to the drum, all clamps in a first set of each pair being operative to clamp the plate at its leading edge onto the surface of the drum and the second set of each pair being operative to clamp the plate at its trailing edge onto the surface of the drum;
and being operative to load the plate from the loading assembly, leading edge first, while the drum rotates in a first direction, and to unload the plate onto the unloading assembly, leading edge first, while the drum rotates in the first direction.
According to further features of the second aspect of the invention, only the first set of clamps clamp the plate during loading and only the second set of clamps clamp the plate during unloading, and each clamp is switchable between two statesxe2x80x94a clamping state and a released statexe2x80x94and circumferentially movable, with respect to the surface of the drum, while in the released state.
According to still further features of the invention, the plate loading assembly and the plate unloading assembly are positioned substantially at one and the same side of the drum or at the same level. In certain configurations of the invention, there are at least two pairs of sets of clamps and the apparatus is further operative to similarly load at least one other plate from the loading assembly and to attach them to the drum in circumferentially tandem positions, and a plate may have any of a plurality of widths and any of a plurality of lengths.
Also disclosed is a method for loading at least one flexible plate onto the cylindrical surface of a rotatable drum, simultaneously clamping each plate thereto and unloading each plate therefrom, each plate having a leading edge and a trailing edge, the method comprising:
(a) providing a plate loading assembly and a plate unloading assembly and positioning them in the vicinity of the drum;
(b) providing, for each plate to be simultaneously clampable to the drum, a pair of sets of clamps and attaching them to the surface of the drum so that each clamp is movable along a circumferential line, whereby each clamp is switchable between a clamping state and a released state and being operative, while in the clamping state, to clamp an edge of a plate engaged thereto onto the surface of the drum;
(c) feeding a plate from the loading assembly until its leading edge engages a first set of a corresponding pair of the sets of clamps, while all clamps thereof are in the released state;
(d) switching the clamps of the first set, while engaged by the plate, to the clamping state, then slowly rotating the drum in a first direction so as to pull the plate from the loading assembly and to wrap it around the drum;
(e) moving the clamps of the second set of the pair, while in the released state, until they engage the trailing edge of the plate wrapped in step (d), then switching them to the clamping state;
(f) repeating steps (c), (d) and (e) for each additional plate to be loaded, if any;
(g) for unloading each clamped plate, switching the clamps of corresponding the first set to the released state and moving them away from the leading edge, then slowly rotating the drum in the first direction so as to push the leading edge of the plate onto the unloading assembly;
(h) switching the clamps of corresponding the second set to the released state, to allow the unloading assembly to pull the plate, released in step (g), from the drum.